Engines may use a turbocharger to improve engine torque and/or power output density. In one example, a turbocharger may include a compressor and a turbine connected by a drive shaft, where the turbine receives exhaust gasses and the compressor receives intake air. In this way, the exhaust-driven turbine supplies energy to the compressor to boost the pressure and flow of air into the engine. For example, boost pressure may be increased by increasing the rotational speed of the turbine. The desired amount of boost may vary over operation of the engine. For example, the desired boost may be greater during acceleration than during deceleration.
One solution to control the boost pressure is a variable geometry turbine. A variable geometry turbine controls boost pressure by varying the flow of exhaust gas through the turbine. For example, exhaust gas may flow from the exhaust manifold through a turbine nozzle and to the turbine blades. The geometry of the turbine nozzle may be varied to control the angle that exhaust gas strikes the turbine blades and/or to vary the cross-sectional area of channels upstream of the turbine blades. Increasing the cross-sectional area of the channels may allow more gas to flow through the channels, but the gas may flow slower compared to gas flowing through a channel with smaller cross-sectional area. The angle of incidence of gas flowing across the turbine blades may affect the efficiency of the turbine, e.g. the amount of thermodynamic energy captured from the flow that is converted to mechanical energy. Thus, the turbine speed and boost pressure may be varied by changing the geometry of the turbine nozzle.
One type of variable geometry turbine includes a swing nozzle vane that pivots within the turbine nozzle. Exhaust gas flowing through the turbine nozzle flows through channels formed between the swing nozzle vanes. Pivoting the vanes in one direction increases the cross-sectional area of channels upstream of the turbine and decreases the incident angle of gas flowing across the turbine blade. Pivoting the vanes in the other direction decreases the cross-sectional area of channels upstream of the turbine and increases the incident angle of gas flowing across the turbine blade. Thus, the swing nozzle vane creates a trade-off between turbine efficiency at high exhaust gas flows or at low exhaust gas flows because the cross-sectional area of the channels and the angle of incidence cannot be varied independently.
Another type of variable geometry turbine uses an annular sliding ring that slides axially in the turbine housing to vary the cross-section of the channels in the turbine nozzle. Thus, an efficient angle of incidence can be maintained over the engine operating range. However, the turbine inlet, at the interface between the turbine nozzle outlet and the turbine blades, is sized for large exhaust flows of the engine. At small exhaust flows, the turbine nozzle channels have a decreased cross-section and energy is lost when the exhaust gas expands from the small turbine nozzle channel to the larger turbine inlet.
The inventors herein have recognized the above issues and have devised an approach to at least partially address them. In one example, an annular turbine nozzle comprises a central axis and a nozzle vane. The nozzle vane includes a stationary vane and a sliding vane. The stationary vane includes a sliding surface in contact with a sliding surface of the sliding vane. The sliding vane is positioned to slide in a direction substantially tangent to an inner circumference of the turbine nozzle. Thus, a desired angle of incidence may be substantially maintained over a range of engine operating conditions. Further, expansion losses may be reduced as compared to a turbine nozzle with an annular sliding ring.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.
At least FIGS. 3-4 are approximately to scale.